UNIVERSITY   OF   CALIFORNIA 

AGRICULTURAL  EXPERIMENT  STATION 
E.   W.    HILGARD,   Director 


CIRCULAR 


A  WARNING  TO  OLIVE  GROWERS. 

FREDERIC  T.  BIOLETTI. 


In  1895  specimens  of  a  serious  disease  of  the  olive  were  received 
at  the  Agricultural  Experiment  Station  from  Merced  County.  This 
disease,  Olive-Knot,  is  common  and  destructive  in  Southern  Europe 
where  it  is  known  as  Tuberculosis  of  the  Olive.  At  the  time  of  its 
discovery  in  Merced  Count}'  the  hope  was  entertained  that  it  had  not 
yet  spread  to  any  other  part  of  California  and  that  it  could  be 
eradicated  by  prompt  and  appropriate  measures.  With  the  purpose 
of  furthering  this  desirable  object  the  horticultural  commissioners 
were  communicated  with  and  a  bulletin  published  describing  the 
disease  and  its  disastrous  effects.* 

Unfortunately  the  hope  entertained  in  1895  has  not  been  realized, 
and  this  year  another  and  larger  infected  area  has  been  discovered  at 
a  long  distance  from  the  first;  and  there  is  strong  reason  to  suspect 
that  there  are  still  others.  An  idea  of  the  destructive  nature  of  the 
disease,  where  the  climatic  conditions  are  favorable  to  its  development, 
is  given  by  the  history  of  the  infected  orchard  in  Merced  County  of 
which  an  account  is  given  in  Bulletin  120.  In  1893  a  single  tree  was 
found  infected  on  one  side  of  the  orchard.  In  1898  several  trees  were 
dead  and  the  disease  had  spread  to  a  large  part  of  the  orchard.  In 
1900  the  orchard  had  become  so  badly  infected  that  it  was  useless  and 


*  Bulletin  120.  The  Olive-Knot. 


was  dug  out.     From  a  single  infected  tree  the  disease  had  passed  in 
seven  years  throughout  the  orchard  and  practically  destroyed  it. 

In  the  infected  area  discovered  this  year  the  conditions  are  some- 
what different.  None  of  the  orchards  are  more  than  three  or  four 
years  old  and  the  trees  were  evidently  infected  before  they  were 
planted;  that  is,  they  were  diseased  when  obtained  from  the  nursery. 
This  is  evidenced  by  the  fact,  vouched  for  by  a  resident  of  the  dis- 
trict who  has  had  exceptional  opportunities  for  observation,  that  the 
trees  from  only  one  nursery  show  evidences  of  disease,  and  that 
practically  alt  the  trees  from  that  nursery  are  affected.  From  the 
appearance  of  the  trees  in  September,  1901,  there  can  be  little  doubt 
that  the  virulence  of  the  disease  in  the  new  locality  will  at  least  equal 
that  displayed  in  the  locality  first  affected.  It  is  practically  certain, 
therefore,  that  none  of  these  young  orchards  where  the  disease  now 
shows  will  ever  come  to  bearing  age,  and  the  sooner  they  are  uprooted 
and  destroyed,  the  better  for  their  owners,  who  will  be  spared  the 
expense  of  maintaining  useless  trees,  and  the  better  for  their  neigh- 
bors, whose  orchards,  by  prompt  action,  may  be  saved  from  infection. 

Destruction  of  Affected  Trees. — There  can  be  no  doubt  in  the 
minds  of  those  who  have  witnessed  the  destructive  effects  of  this 
disease  in  warm  climates  that  any  young  trees  showing  undoubted 
symptoms  of  Olive-Knot  in  such  climates  should  be  destroyed  imme- 
diately. The  same  treatment  would  undoubtedly  be  the  wisest  in  the 
case  also  of  old  trees  or  of  affected  trees  in  cooler  localities  in  Cali- 
fornia, if  we  consider  the  interests  of  the  olive- growing  industry  as  a 
whole;  for  though  the  Knot  may  have  and  probably  has  extended  to 
several  localities  already,  the  policy  of  destroying  the  affected  trees  as 
soon  as  found  will  certainly  retard  its  spread,  even  though  its  total 
eradication  is  impossible.  Whether  the  attempted  eradication  is 
carried  out  by  county  horticultural  commissioners  or  by  the  pro- 
prietors of  the  orchards  themselves,  the  question  will  arise  whether 
only  those  trees  which  show  undoubted  knots  should  be  destroyed,  or 
whether  the  uprooting  and  burning  should  be  extended  to  all  the  trees 
in  an  affected  orchard.  It  would  be  extremely  expensive  and  probably 
useless  to  adopt  the  latter  course  if  the  disease  had  extended  widely  to 
many  localities,  and  in  many  old  orchards.  As,  however,  but  a  very 
small  percentage  of  the  old  orchards  are  at  present  affected  and  as  the 
young  affected  orchards  will  never  be  of  any  value,  the  latter  course  is 
undoubtedly  the  wisest  to  adopt  and  the  safest  for  the  olive- growers 
as  a  whole.  All  the  trees  in  young  orchards  which  are  just  beginning 
to  show  knots,  and  which  were  probably  infected  in  the  nursery  from 
which  they  came,  are  probably  diseased.  A  grower  writes  to  the 
Agricultural  Experiment  Station  regarding  a  young  orchard  where 


only  a  part  of  the  trees  showed  knots  above  ground:  "I  dug  up  several 
trees  that  I  had  reason  to  suspect,  but  which  showed  nothing  above 
ground.  These  too  were  found  a  mass  of  knots."  Even  though  some 
of  the  trees  in  such  an  orchard  do  not  contain  the  germs  of  the  malady, 
it  is  impossible  to  pick  them  out  with  any  certainty ;  and  if  but  one  or 
two  infected  trees  are  left  they  will  sooner  or  later  infect  the  whole 
orchard.  It  is  moreover  useless  to  plant  young  trees,  even  healthy 
ones,  in  places  from  which  diseased  trees  have  been  taken.  Such 
replants  cannot  thrive  and  will  be  literally  covered  with  knots  the 
first  year  after  planting. 

In  view  of  the  serious  nature  of  this  disease,  which  threatens  to 
make  olive  growing  impossible  in  some  of  the  most  favorable  localities 
of  the  State,  it  behooves  the  olive  growers  to  cooperate  heartily  with 
the  horticultural  commissioners  in  hunting  out  and  destroying  all 
cases  of  the  disease  wherever  it  occurs.  It  is  especially  necessary 
that  every  olive  nursery  should  be  carefully  examined  and  no  one 
should  plant  young  trees  from  a  nursery  before  they  have  satisfied 
themselves  by  personal  inspection,  or  have  received  a  satisfactory 
guaranty  from  the  nurserymen  or  the  county  horticultural  com- 
missioner that  the  nursery  is  free  from  any  case  of  the  disease.  The 
disease  is  highly  infectious,  go  that  a  tree  apparently  healthy  but 
growing  near  a  tree  which  shows  signs  of  the  disease  almost  certainly 
contains  the  germs  of  the  disease  itself. 

Those  who  are  unfamiliar  with  the  symptoms  can  obtain 
Bulletin  120  by  applying  to  the  Agricultural  Experiment  Station 
at  Berkeley.  In  this  bulletin  the  disease  is  described  with 
sufficient  detail  to  enable  anyone  to  recognize  it.  In  buying 
young  trees  for  planting  it  is  well  to  remember  that  it  is  possible 
for  the  nurseryman  to  remove  the  knots  from  slightly  affected  trees, 
so  that  they  may  appear  healthy  when  received  by  the  purchaser. 
Such  trees  will  almost  certainly  develop  the  disease  later,  after  plant- 
ing in  the  orchard.  The  only  safe  method  is  to  examine  the  trees  in 
the  nursery,  preferably  in  July  or  August,  at  which  time  the  knots  are 
growing  rapidly,  making  it  impossible  to  disguise  the  disease  in  a 
badly  affected  nursery. 


Berkeley,  October  10,  1901 


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2  1  3 

1.  Young  tree  planted  to  replace  one  destroyed  by  knot.     Photograph 7taken 

twelve  months  after  planting. 

2.  Large  knots  on  still  vigorous  branch  of  old  tree. 

3.  Small  knots  on  much  weakened  branch. 


